Ryan Moore opened with five birdies in seven holes and never missed a fairway after the first one, posting a 7-under 65 for his best start in his 14th appearance at the Memorial. He was one shot ahead of Jordan Spieth, who chipped in for birdie, chipped in for par and holed a 35-foot eagle putt.
Tiger Woods made a pair of late birdies to salvage a 70 in his first round since missing the cut at the PGA Championship. He played his back nine in a foursome with Bryson DeChambeau, Justin Rose and a rules official in a cart timing them because they were so far out of position.
Getting caught up wasn't easy with various tee shots in water hazards, though it was obvious how far behind they were. Spieth, Rory McIlroy and Justin Thomas were in the group ahead of them, and McIlroy hit his tee shot on the par-4 second into a backyard. With no official nearby, he had to walk 300 yards back to the tee to hit again. That took time. Still, walking off the fourth green, the group of Woods, DeChambeau and Rose still had not reached the third tee.
DeChambeau, who considers such variables as air density and elevation change in his pre-shot routine, went over his allotted time on No. 5 and was given a warning for a bad time. He made birdie, took double-bogey from a fairway bunker on the next hole and began his title defense with a 74.
Golf still is measured by score, and Moore had the lowest on a rain-softened Muirfield Village in Dublin, Ohio. Only two of his seven birdies were longer than 10 feet, and the only time he came close to a bogey was on his opening hole, where he saved par with a 6-foot putt.
He was among 22 players who broke 70, and only 44 players broke par despite the soft conditions. Phil Mickelson, using two drivers this week to go after longer tee shots on a half-dozen holes, opened with a 70.
Spieth looked as though he couldn't miss for the longest time. On his second hole, the par-5 11th, his wedge came up so short on a soft green that it spun off the front. He chipped in from 50 feet for birdie. Another chip from thick rough caught the slope on the back of the par-5 15th green and rolled down to 3 feet for a birdie.
He went out in 32, made an 8-foot birdie putt on No. 3 and then had consecutive holes that illustrated how his round was going.
On the par-3 fourth, his tee shot was buried in the slope of a mound above the bunker. With his feet well below the ball, he hooked it out onto and across the green into more rough, and then chipped in for par. On the par-5 fifth, his hybrid caught the right side of the green and he rolled in the long eagle putt.
Spieth took only 22 putts for the round.
And then his luck ran out with a tee shot that plugged into the sand left of the green on the par-3 eighth, leaving him two options: go at the pin and run off the green into rough, or aim away from the flag and leave a 60-foot putt for par. He chose the latter and came inches within making it.
"Sooner or later, it was going bite me," Spieth said with a smile.
Even so, he had no complaints.
"Six under around Muirfield I'd take any day of the week, no matter what form you're coming into it with," he said. "I felt like I hit more fairways today, gave me some more opportunities, and the putter stayed hot."
Thomas, in his first tournament since the Masters because of a bone bruise in his right wrist, showed plenty of rust in his round of 71. McIlroy had a 75 with two double-bogeys, both from tee shots either lost (No. 15) or out of bounds (No. 2).
Anirban Lahiri, Marc Leishman and Martin Kaymer were at 67.
Woods made birdies on all but one of the par 5s. His regret was a few loose iron shots that led to bogey, especially on the 13th when he hit 9-iron from the fairway into a bunker that led to a careless bogey. But he finished strong -- eventually -- and while 10 players from his side of the draw broke 70, he wasn't too far behind.
At least on the leaderboard.
"That was frustrating, because the last eight holes we were on the clock," Woods said. "The group ahead of us ... JT doesn't take a lot of time, Rory plays quick and Jordan was 7 under. So they were obviously playing fast. And we were obviously not."
The Associated Press contributed to this report.